1. Technical Field of the Invention
The present invention relates to the transmission of duplicate communications and, in particular, to the synchronous reception of the duplicate communications. The present invention more specifically relates to handoff within a cellular telephone system and, in particular, to a method and apparatus for effectuating a seamless soft handoff within a code division multiple access (CDMA) cellular communications system wherein the call subject to handoff utilizes duplicate downlink communication transmissions.
2. Description of Related Art
System configuration and operation of a code division multiple access (CDMA) cellular communications system is well known to those skilled in the art. Accordingly, detailed information concerning CDMA system configuration and operation is not provided. However, technical information concerning this topic may be obtained by referring to a number of available documents. For example, for a description of the use of CDMA techniques in a multiple access communications system, reference is made to U.S. Pat. No. 4,901,307, entitled "Spread Spectrum Multiple Access Communication System Using Satellite or Terrestrial Repeaters". Furthermore, for a description of the generation of signal waveforms for use in a CDMA communications system, reference is made to U.S. Pat. No. 5,103,459, entitled "System and Method for Generating Signal Waveforms in a CDMA Cellular Telephone System". Finally, for an identification of industry standards relating to CDMA cellular communications systems, reference is made to TIA/EIA Interim Standard IS-95-A dated May, 1995. The disclosures of the foregoing references are expressly incorporated by reference herein.
In cellular communications systems of many types (for example, frequency division multiple access (FDMA) systems such as the Advanced Mobile Phone System (AMPS), or combined FDMA and time division multiple access (TDMA) systems such as the Global System for Mobile (GSM) communications, or code division multiple access (CDMA) systems such as that specified by IS-95-A), a handoff scheme of some sort is implemented to allow a cellular call to continue in instances when a mobile station crosses the boundary between two cells. In one commonly used handoff scheme, handoff is initiated when received signal strength in a given cell (or sectors if cell is split, with each sector considered orthogonal) for mobile station communication falls below a given threshold. The system then determines whether a neighboring cell can receive mobile station communications with a greater signal strength. If such a cell is identified, that cell is selected for the handoff. In another commonly used handoff scheme, the mobile station additionally or alternatively makes downlink signal strength measurements with respect to the given cell and its neighboring cells. These measurements are reported to the system for use in making the handoff determination and selecting the cell to which handoff occurs. In either of these schemes, the system informs the mobile station as to which cell is to be used in the future for continuing the call, and a corresponding re-routing of the call to that cell is made.
In conventional FDMA or FDMA/TDMA type cellular communications systems, each (adjacent) cell operates with a different set of radio frequencies. Accordingly, for each of the foregoing handoff schemes, the handoff must comprise a "hard" handoff. By this it is meant that the mobile station must change transceiver operation from a radio frequency used in and assigned to the current cell to a radio frequency used in and assigned to the new cell. Due to the limited functionalities of the transceiver for the mobile station, the mobile station must break its communication link with the currently serving cell before making the communication link with the newly serving cell. This "break before you make" hard handoff operation presents some serious operational concerns because it is especially susceptible to loss of information during the time elapsed between the "break" of the old link and the "make" of the new link.
In CDMA communications systems, however, the same frequency band can be used for all cells. Accordingly, for each of the foregoing handoff schemes, the handoff may comprise a "soft" handoff. By this it is meant that the mobile station needs not necessarily change frequencies when switching communications from the current cell to the new cell. Communication with the mobile station is often simultaneously effectuated by two or more cells using the same frequency band, with the call switched between the two or more cells as measured signal strength dictates. When the signal strength measurements confirm a complete mobile station transition into the newly serving cell, communications utilizing the prior cell are discontinued. This "make before you break" soft handoff operation is less susceptible to instances of loss of information because the mobile station is continuously in communication with at least one cell throughout the handoff process.
Operation of a code division multiple access cellular communications system to implement soft handoff operation is well known to those skilled in the art. Accordingly, detailed information concerning conventional CDMA soft handoff processes is not provided. However, technical information concerning this topic may be obtained by referring to a number of available documents. For example, for a description of a system for controlling soft handoff in a CDMA system, reference is made to U.S. Pat. No. 5,101,501, entitled "Method and System for Providing a Soft Handoff in Communications in a CDMA Cellular Telephone System". Furthermore, for a description of a system implementing mobile assisted soft handoff, reference is made to U.S. Pat. No. 5,267,261, entitled "Mobile Station Assisted Handoff in a CDMA Cellular Communications System". Still further, for a description of the use of a packet switching architecture to support, among other operations, a CDMA soft handoff, reference is made to U.S. Pat. No. 5,305,308, entitled "Wireless Access Telephone-To-Telephone Network Interface Architecture". The disclosures of the foregoing references are expressly incorporated by reference herein.
Handoff generally occurs with respect to one of two potential cell configurations. In a first configuration, see FIG. 1A, a cell 10 is divided into a plurality (typically three) sectors 12, each having a different associated antenna 14, but served by a single base station (BS) 16. As a mobile station 18 moves in the direction of arrow 20, it may pass from the radio coverage area of a first sector 12(1) of the cell 10 to the radio coverage area of a second sector 12(2) of the same cell. In such a case, and in accordance with the make before you break handoff process, the mobile station 18 simultaneously communicates with the base station 16 utilizing both a first antenna 14(1) and a second antenna 14(2), respectively. The duplicate frames of data simultaneously communicated between the mobile station 18 and base station 16 utilizing both antenna 14(1) or 14(2) and their associated communications paths 22(1) or 22(2) arrive at the intended destination substantially simultaneously (i.e., within microseconds of each other). This is important because synchronized processing of the duplicate transmitted frames is essential to proper soft handoff operation and continued processing of the call.
In a second configuration, see FIG. 1B, synchronization of frame processing becomes more problematic. Two cells 10 (possibly each including multiple sectors--not shown) are illustrated each served by a different base station 16 having an antenna 14. The base stations are connected to a mobile switching center (MSC) 24. As the mobile station 18 moves in the direction of arrow 20, it may pass from the radio coverage area of a first cell 10(1) to the radio coverage area of a second cell 10(2). In such a case, and in accordance with the make before you break handoff process, the mobile station 18 simultaneously communicates with the mobile switching center 24 utilizing both the first base station 16(1) and the second base station 16(2). The duplicate frames of data simultaneously communicated between the mobile station 18 and base station 16 utilizing either base station 16(1) or 16(2) and their associated communications paths 22(1) or 22(2) are assumed to arrive at the intended destination substantially simultaneously. However, due to differences in propagation times over the paths 22, as well as differences in transmission delay between the mobile switching center 24 and the base stations 16, the frames may not arrive substantially synchronously and in fact may be offset in time from each other by many milliseconds.
A CDMA frame may have a duration as short as 20 ms. If the encountered offset exceeds this frame duration, then the simultaneously transmitted duplicate frames may arrive at the destination a full frame or more apart from each other. Depending on the duration of this offset, synchronized processing of the duplicate transmitted frames may be difficult, if not impossible, and proper soft handoff operation impeded to a degree that potentially compromises call continuation.
Accordingly, the second configuration soft handoff of FIG. 1B requires the following: (1) routing of duplicate mobile station communications through different cells to a common call processing point (such as a mobile switching center); (2) simultaneous (i.e., synchronous) processing of the duplicate mobile station communications at the common call processing point; (3) routing of duplicate common call processing point communications through different cells to a mobile station; and (4) simultaneous (i.e., synchronous) processing of the duplicate common call processing point communications at the mobile station. First configuration soft handoff requires substantially the same operational characteristics except that the duplicate communications are instead routed through different antennas of the same cell (base station) rather than through different cells. Difficulties with synchronous processing are not, however, typically a concern because signal transmission times in FIG. 1A are nearly identical. Such is not the case, however, with FIG. 1B. There is a need then for a method and apparatus for facilitating synchronous processing of the duplicate communications in those instances where the signal transmission times of the duplicate communications are not substantially the same.